Bias and variance of extrapolated tails for area-under-the-curve (AUC) and area-under-the-moment-curve (AUMC).

The usual formulas for the extrapolated values of AUC and AUMC from tn to infinity produce estimates that are biased. An empirical correction factor is shown to reduce both bias and variance of estimates. The fitted value Cz at the last time-point tn should normally be used in the formulas, rather than the measured concentration Cn. Nonlinear least squares methods offer no advantage over the fitting of a straight line to ln (C).